This invention relates to key telephone systems and, more particularly, to such systems employing or which are capable of employing nonlocking key telephone sets.
Most present-day key telephone systems continue to employ key telephone sets of the locking key variety. The ubiquitous models 565HK (rotary dial) and 2565HK (pushbutton dial) telephone sets manufactured by the Western Electric Company are of this type. These sets include a hold button and a plurality of pickup key buttons for allowing the telephone user to access up to five different central office lines or trunks. When a pickup key is depressed to access a particular line, a mechanical detent mechanism associated with the key button group causes the button to latch in its depressed or operated condition. The station user is thus given a mechanical indication of the line that has been accessed by the set because that line is identified by the depressed key button. In addition, the pickup key buttons may be equipped with lamps.
More recently, nonlocking key telephone sets have been devised in which the mechanical detent mechanism has been eliminated. Because there is no mechanically latched button to identify the line being accessed by the station set, sets equipped with nonlocking key buttons must be provided with lamps to indicate to the user the status of the lines terminating at the station. An example of a prior art system employing nonlocking key telephone sets is disclosed in H. P. Anderson et al U.S. Pat. No. 3,244,815, issued Apr. 5, 1966.
Most prior art circuits responsible for key lamp illumination, such as those disclosed in R. E. Barbato et al U.S. Pat. No. 3,436,488, issued Apr. 1, 1969, and C. E. Morse et al U.S. Pat. No. 3,239,610, issued Mar. 8, 1966, have typically made no distinction in the illumination or interruption rate delivered to the different key telephone sets in a group. For example, if a given key telephone set has access to four different lines one of which is picked up, one of which is ringing, one of which is on HOLD, and one of which is idle, the key lamp for the first line will be steadily illuminated at all telephone sets having a pickup key for this line, the key lamps for the second line will all be illuminated at the ringing rate, those for the third line will all be illuminatad at the hold rate, and the lamps for the last line will all be dark. One exception to this has been the "I-HOLD" system of lamp illumination disclosed in A. D. Limiero et al U.S. Pat. Re. 26,722, issued Nov. 25, 1969, in which a distinctive lamp illumination was provided at the set placing the line on hold while conventional hold illumination was provided at all other sets capable of accessing the held line.
While the aforementioned systems of uniform key lamp illumination are quite satisfactory for illuminating the key buttons of conventional locking key telephones sets, the great profusion of winking, blinking, and steadily illuminated lamps can be confusing when there is nothing else (e.g., a locked key) to help focus the station user's attention. Further confusion for a user scanning the pickup keys of a nonlocking set may result from the use of low current, light emitting diodes to illuminate the nonlocking keys since these devices do not presently provide as bright or as extensive illumination as the conventional incandescent bulbs.
It is therefore an object of the present invention to provide clear and easily distinguishable line status information in nonlocking key telephone sets under a variety of lamp interruption rates and ambient lighting conditions in which the set may be employed.
Alternate schemes for more clearly displaying line status information provide for the use of more than one lamp per key. However, the additional lamps and related control circuitry are expensive and complex.
It is an object of the present invention, therefore, to provide complete line status information in nonlocking key telephone sets inexpensively and simply.